Ceramics and ceramic-metal composite (i.e., cermet) are usually formed by powder metallurgical methods. A ceramic is an inorganic nonmetallic material, such as an oxide, carbide, nitride, boride, silicide or alloy thereof. A cermet is a material containing a ceramic and a free metal, such as a cemented tungsten carbide. In forming a ceramic or cermet, a powder or powder mixture is shaped into a porous body (i.e., greenware) by techniques, such as pressing or slip casting. The greenware is then heated to remove volatile materials, such as water and plastic binders, and, subsequently, heated to a higher temperature to form a dense body. The greenware typically displays a linear shrinkage of about 15 percent. The shrinkage may vary within the part due to differing powder compaction, such as between regions having significantly different cross-sectional areas. When the shrinkage between two areas is too large, the part tends to crack during heating.
Several methods have been employed to make complex shaped cermet and ceramic bodies via powder metallurgical techniques. Techniques that have been employed include machining of the greenware (e.g., spark plugs) and injection molding (e.g., turbocharger rotors). Greenware machining causes significant loss of material and, generally, cannot form highly complex or small parts due to, among other things, the fragility of the greenware body. Injection molding requires expensive tooling and heating of the molded body for long times to remove plastic additives needed to form the body.
Recently, U.S. Pat. No. 5,333,520 disclosed complexed shaped cermets made by uniaxially pressing two or more shapes that are placed upon each other with the joint between them lying essentially horizontally and, subsequently, sintering to form a complex shaped body. The two bodies may be of differing composition, but when the compositions are different, the shrinkage must be similar to avoid breakage, thus limiting the body compositionally.
Composite structures of metal, ceramics and cermets have also been formed by pressing, in a die, differing powder compositions to form a composite green body (shaped porous powder compact). The composite green body has regions that are comprised of different powder compositions. The green body is then sintered to form a dense body. Examples of this technique are described in U.S. Pat. Nos. 4,329,175; 4,372,404 and 4,602,956.
Because each of the techniques described above requires the sintering of a powder compact having differing compositions, the interface between regions in the body is dictated by the sintering. Because of the time and temperature it takes to sinter, the interface between regions tends to be diffuse. The variation of composition is also limited by the sintering shrinkage between the differing powder compositions.
Therefore, it would be desirable to provide a method of forming a complex shaped or composite body of a metal, ceramic, cermet or combination thereof that avoids one or more of the problems of the aforementioned methods.